This invention relates to apparatus and a method for the manufacture of continuous fibers or filaments from mineral thermoplastic materials such as glass, obtained by mechanical attenuation of the molten material flowing from a plurality of orifices situated at the base of a bushing. The invention is especially concerned with a process for the manufacture of glass threads or strands composed of a plurality of filaments of differing cross sections.
The filaments generally obtained by mechanical attenuation of streams of glass issuing from a plurality of identical orifices normally have substantially identical circular cross sections due to the high surface tension of the molten glass. Consequently, when these filaments are gathered together to form a thread, the filaments are separated by numerous gaps which substantially reduce the compactness of the said thread.
The use of such a thread as reinforcing elements for thermosetting or thermoplastic synthetic organic materials does not enable as much glass to be incorporated in these materials as would be desirable for improving their mechanical properties. This limitation may be overcome by assembling filaments of differing cross-sectional shape or filaments whose cross-sectional areas are identical but not circular.
The manufacture of filaments having non-circular cross sections has been described in particular in the article by R. A. Humphrey, entitled "Forming Glass Filaments with Unusual Cross Sections" (7th International Glass Conference - Brussels 1965).
According to the above-mentioned article, filaments of rectangular or hexagonal cross section, for example, may be obtained by a process similar to that used for the manufacture of optical fibers.
A rod or preform of the desired cross section is used as starting material, and its lower end is softened in order to draw it out into a sufficiently fine filament in which the initial cross section is preserved on a reduced scale.
This process requires the previous manufacture of the preform and very precise control of the forming temperatures. Its discontinuous and small scale character does not enable it to be envisaged for exploitation on a large industrial scale.
Another process is called to mind in the description given in U.S. Pat. Nos. 2,758,951 and 2,906,656 relating to the manufacture of a composite material.
The apparatus for carrying out the process described in these patents comprises a bushing provided in its lower part with two series of different circular orifices alternating regularly in a single longitudinal row. A plurality of filaments of circular contour with differing cross sections is attenuated from these orifices in such a manner as to form a sheet of parallel filaments wherein each filament of large diameter is surrounded by several filaments of smaller diameter. The sheet is rolled up on a drum on which are formed up to several hundreds of successive layers. Apart from an approximate indication of the diameter of the orifices, no precise information is given as to the operating conditions of such a bushing.
The French patent published under the No. 1,341,710 describes a process for the production of filaments having a non-circular cross section from a bushing with conventional tips.
This process consists in establishing contact between the surface of the glass as it leaves the orifice and a cooled surface. The viscosity of the glass increases locally under the cooling effect so that the resulting deformation of the glass becomes set. A wide variety of filaments with non-circular cross section can be obtained in this manner.
Apart from the difficulties of operating such an installation, it is obvious that the number of filaments delivered from each bushing is very small since each tip must be equipped with a deformation device which takes up a far from negligible amount of space. Such a solution is only of limited industrial interest.
The present invention has as its object a fiberizing apparatus and process for obtaining a continuous strand or thread composed of a large number of filaments of differing cross sections.
It is a particular object of the present invention to obtain from a given fiberizing apparatus a thread composed of a mixture of filaments of differing cross sections in constant and definite proportions chosen so as to increase the degree of compactness of the said thread.
These objects are achieved by the use of a novel form of bushing in a fiberizing installation as defined below.
The fiberizing installation essentially comprises a source of supply of thermoplastic material such as glass connected to a bushing heated by Joule's effect, a device for blowing gas directed towards the bottom of the bushing, and attenuating means, the lower surface of the bottom of said bushing having a multiplicity of fiberizing centers of differing configuration, each fiberizing center being formed by a zone perforated by a group of orifices from which at least one filament may be attenuated in a stable manner.
By "fiberizing centers of differing configurations" are meant zones perforated by groups of orifices which are distinguished from each other, in one or more ways, such as the nature of the metal alloy of which they are constituted and/or by the number of orifices and/or by the cross section of the orifices and/or by the distance separating the orifices.
According to one characteristic of the invention, these fiberizing centers are regularly arranged on the bottom of a bushing consisting of a substantially continuous plate.
According to another characteristic of the invention, these fiberizing centers are arranged at the bottoms of bosses provided on the bushing, the walls of which bosses encircle at least one alveole or internal cell.
Starting from a bushing having fiberizing centers of differing configuration, the manufacturing process according to the invention consists of regulating the heating of the bushing and/or cooling the bottom of said bushing in such a manner as to bring the glass to a temperature at which the formation of one filament per orifice in a certain number of predetermined fiberizing centers takes place simultaneously with the formation of one filament per group of orifices in other fiberizing centers.